Structure and structure-function relationships in glycoprotein hormones

Profile of follitropin alpha/lutropin alpha combination for the stimulation of follicular development in women with severe luteinizing hormone and follicle-stimulating hormone deficiency

A severe gonadotropin deficiency together with chronic estradiol deficiency leading to amenorrhea characterizes patients suffering from hypogonadotropic hypogonadism. Administration of both follicle-stimulating hormone (FSH) and luteinizing hormone (LH) to these patients has been shown to be essential in achieving successful stimulation of follicular development, ovulation, and rescue of fertility. In recent years, the availability of both recombinant FSH (rFSH) and recombinant LH (rLH) has provided a new therapeutic option for the stimulation of follicular growth in hypopituitary–hypogonadotropic women (World Health Organization Group I). In this article, we review the data reported in the literature to highlight the role and the efficacy of using recombinant gonadotropins, rFSH and rLH, in the treatment of women with severe LH/FSH deficiency. Although the studies on this issue are limited and the experiences available in the literature are few due to the small number of such patients, it is clearly evident that the recombinant gonadotropins rFSH and rLH are efficient in treating patients affected by hypogonadotropic hypogonadism. The results observed in the studies reported in this review suggest that recombinant gonadotropins are able to induce proper follicular growth, oocyte maturation, and eventually pregnancy in this group of women. Moreover, the clinical use of recombinant gonadotropins in this type of patients has given more insight into some endocrinological aspects of ovarian function that have not yet been fully understood.

Discovery and Development of Small Molecule Allosteric Modulators of Glycoprotein Hormone Receptors

Glycoprotein hormones, follicle-stimulating hormone (FSH), luteinizing hormone (LH), and thyroid-stimulating hormone (TSH) are heterodimeric proteins with a common α-subunit and hormone-specific β-subunit. These hormones are dominant regulators of reproduction and metabolic processes. Receptors for the glycoprotein hormones belong to the family of G protein-coupled receptors. FSH receptor (FSHR) and LH receptor are primarily expressed in somatic cells in ovary and testis to promote egg and sperm production in women and men, respectively. TSH receptor is expressed in thyroid cells and regulates the secretion of T3 and T4. Glycoprotein hormones bind to the large extracellular domain of the receptor and cause a conformational change in the receptor that leads to activation of more than one intracellular signaling pathway. Several small molecules have been described to activate/inhibit glycoprotein hormone receptors through allosteric sites of the receptor. Small molecule allosteric modulators have the potential to be administered orally to patients, thus improving the convenience of treatment. It has been a challenge to develop a small molecule allosteric agonist for glycoprotein hormones that can mimic the agonistic effects of the large natural ligand to activate similar signaling pathways. However, in the past few years, there have been several promising reports describing distinct chemical series with improved potency in preclinical models. In parallel, proposal of new structural model for FSHR and in silico docking studies of small molecule ligands to glycoprotein hormone receptors provide a giant leap on the understanding of the mechanism of action of the natural ligands and new chemical entities on the receptors. This review will focus on the current status of small molecule allosteric modulators of glycoprotein hormone receptors, their effects on common signaling pathways in cells, their utility for clinical application as demonstrated in preclinical models, and use of these molecules as novel tools to dissect the molecular signaling pathways of these receptors.

Trichostatin A specifically stimulates gonadotropin FSHβ gene expression in gonadotroph LβT2 cells

Trichostatin A (TSA) is a selective inhibitor of mammalian histone deacetylase. In the present study, TSA was found to selectively increase gene expression of the pituitary gonadotropin β-subunit of follicle-stimulating hormone (FSH). Stimulation of mouse pituitary gonadotroph cell lines, LβT2, with TSA for 24 h resulted in no change in mRNA expression of the α- and LHβ-subunit. On the other hand, FSHβ-subunit mRNA expression was significantly increased in a dose-dependent fashion. Similarly, specific induction of the FSHβ-subunit gene with TSA stimulation was observed in primary cultures of rat pituitary cells. Histone acetylation in whole cell lysates of LβT2 cells was significantly increased after TSA treatment, but not gonadotropin-releasing hormone (GnRH) treatment. The effect of TSA on FSHβ mRNA expression was prominent compared to that of GnRH; however, TSA-stimulated FSHβ mRNA expression was significantly reduced with combined TSA and GnRH treatment. TSA caused a slight increase in extracellular signal-regulated kinase (ERK) phosphorylation, while GnRH-increased ERK phosphorylation was potentiated in the presence of TSA. In addition, TSA, but not GnRH, significantly stimulated gene expression of retinaldehyde dehydrogenase 1 (RALDH1), a retinoic acid (RA) synthesizing enzyme involved in cell differentiation. These findings demonstrate that TSA specifically increases FSHβ subunit gene expression with a concomitant increase in whole cell histone acetylation. Moreover, although GnRH is a stimulator of FSHβ gene expression, it interfered with the stimulatory effect of TSA on FSHβ mRNA expression, without modification of TSA-increased whole cell histone acetylation. This suggests that the mechanisms of TSA and GnRH-induced gonadotropin subunit gene expression are entirely distinct.

Distinct expression patterns of glycoprotein hormone subunits in the lophotrochozoan Aplysia: implications for the evolution of neuroendocrine systems in animals

Glycoprotein hormones (GPHs) comprise a group of signaling molecules critical for major metabolic and reproductive functions. In vertebrates they include chorionic gonadotropin, LH, FSH, and TSH. The active hormones are characterized by heterodimerization between a common α and hormone-specific β subunit, which activate leucine-rich repeat-containing G protein coupled receptors. To date, genes referred to as GPHα2 and GPHβ5 have been the only glycoprotein hormone subunits identified in invertebrates, suggesting that other GPHα and GPHβ subunits diversified during vertebrate evolution. Still the functions of GPHα2 and GPHβ5 remain largely unknown for both vertebrates and invertebrates. To further understand the evolution and putative function of these subunits, we cloned and analyzed phylogenetically two glycoprotein subunits, AcaGPHα and AcaGPHβ, from the sea hare Aplysia californica. Model based three-dimensional predictions of AcaGPHβ confirm the presence of a complete cysteine knot, two hairpin loops, and a long loop. As in the human GPHβ5 subunit the seatbelt structure is absent in AcaGPHβ. We also found that AcaGPHα and AcaGPHβ subunits are expressed in larval stages of Aplysia, and we present a detailed expression map of the subunits in the adult central nervous system using in situ hybridizations. Both subunits are expressed in subpopulations of pleural and buccal mechanosensory neurons, suggesting a neuronal modulatory function of these subunits in Aplysia. Furthermore it supports the model of a relatively diffuse neuroendocrine-like system in molluscs, where specific primary sensory neurons release peptides extrasynaptically (paracrine secretion). This is in contrast to vertebrates and insects, in which releasing and stimulating factor from centralized sensory regions of the central nervous system ultimately regulate hormone release in peripheral glands.

Molecular evolution of the polypeptide hormones

Any biological function is at least bimolecular and its evolution therefore is at least dual, with variations in two lines of molecules. The hormone specificity results from a particular fit between the three-dimensional structure of the agent and that of the receptor but, because receptors are not known at the structural level, a discussion on the evolution of the polypeptide hormones is mainly limited to the possible progressive changes of the latter. As for other proteins (enzymes, oxygen carriers etc.) two degrees of complexity can be distinguished according to whether the hormone comprises one or several polypeptide chains. Protein assembly can bring new biological properties, each subunit playing a particular role. In this case, the 'internal' evolution (chain-chain interactions) overlaps the 'external' evolution (hormone-receptor contacts). The 'monomeric' hormones present the following problems: evolution of the prohormone and of the converting enzyme (for insulin), duplication and differentiation of two lines of hormones either by amino acid substitutions (neurohypophysial hormones and neurophysins) or by substitutions and size modifications (corticotropin and lipotropin), duplication and fusion leading to internal homology in the single polypeptide chain (somatotropin, prolactin, placental lactogen). The 'dimeric' hormones lead to several problems: successive duplications giving different subunits, selective associations between subunits, unequal rates of evolution of the subunits, the function of each subunit (lutropin, follitropin, thyrotropin, choriogonadotropin). An attempt is made to integrate the evolution of polypeptide hormones in the frame of the evolution of proteins.